I have looked at quite a few of these tools and Enterprise architect is my favourite.
It includes comprehensive support for the new UML 2.0 standard and for all 13 UML 2.0 diagrams.
You can download a months free trial. The commercial version is good value at only USD189.
See:
http://www.sparxsystems.com.au [ October 22, 2004: Message edited by: Patrick Cogan ]